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THE AMERICAN NATURALIST 



[Vol. XLV 



uated near the posterior end of the ventral groove. Then 

 they separate into two apparently equal groups, one on 

 cither side of the embryo, which are soon recognizable as 

 the two germ glands. 



Because only those cells which gather in the pole disc 

 granules become germ cells, I have called these granules 

 "germ cell determinants" (Hegner, '08). This term has 

 been objected to by Wieman ( '10) because "the term im- 

 plies the attribute of certain potentialities that these 

 granules have not been shown to possess" (p. 180). The 

 morphological evidence is, I believe, strong enough to 

 warrant the use of the term; recent experiments, how- 

 ever, add to the convincing facts already published 

 (Hegner, '08, '09). It is possible to show that if the pole 

 disc is prevented from taking part in the development 

 of the egg, no germ cells will be produced. Attempts to 

 extract the pole disc by means of pricking the freshly laid 

 egg and allowing them to flow out were only partially 

 successful (Hegner, '08). A new method was later em- 

 ployed which absolutely prevented the cleavage nuclei 

 from encountering the pole disc. In these experiments 

 the posterior end of the egg was touched with a hot needle 

 and that portion containing the pole disc was killed. In 

 every instance the development continued and in the eggs 

 so far examined the blastoderm formed normally over 

 all of the surface except at the posterior end; here it was 

 built at the end of the living substance as shown in Fig. 

 5, F, bl. No germ cells were produced. I conclude from 

 this that the pole disc granules are necessary for the 

 formation of germ cells, and that they are really "germ 

 cell determinants." Of course it might be argued that 

 some other substance lying at the posterior end of the 

 egg is responsible for the differentiation of the germ 

 cells, but this seems highly improbable. Wieman 

 ('10) states that in Leptinotarsa signaticollis, a 

 species I have not studied, "the granules are not all 



part of them remains behind after the cells have passed 

 through" (p. 186). This is certainly not the case in the 

 many eggs that I have examined, and a reexamination 



